This application is based on application No. H9-257204 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly to a high-aperture-ratio and high-magnification zoom lens system suitable for photographic cameras and video cameras.
2. Description of the Prior Art
Conventionally, for use in photographic cameras and video cameras, various zoom lens systems have been proposed that consist of four lens units that are, from the object side, a first lens unit having a positive optical power, a second lens unit having a negative optical power, a third lens unit, and a fourth lens unit. For instance, Japanese Laid-Open Patent Application No. H4-369611 proposes a zoom lens system consisting of four lens units that are, from the object side, a first lens unit having a positive optical power, a second lens unit having a negative optical power, a third lens unit, and a fourth lens unit, with at least one of these lens units including a gradient-index lens element (i.e. a lens element having an index distribution).
However, this zoom lens system has the following disadvantage: whereas the number of lens elements is successfully reduced by the use of a gradient-index lens element in some of the lens units, an unduly large number of lens elements are required in the second lens unit to obtain a strong optical power as desired there.
An object of the present invention is to provide a high-performance zoom lens system that, despite providing a high aperture ratio and a high magnification, consists of as few constituent lens elements as possible.
To achieve the above object, according to one aspect of the present invention, a zoom lens system is provided with, from the object side, a first lens unit having a positive optical power, a second lens unit having a negative optical power, a third lens unit, and a fourth lens unit. Additionally, this lens system fulfills the condition shown below:
0.4 less than |xcfx862/xcfx86W| less than 0.8
where
xcfx862 represents the optical power of the second lens unit; and
xcfx86W represents the optical power of the entire zoom lens system at the wide-angle end.
According to another aspect of the present invention, in the above zoom lens system, at least one of the first to fourth lens units includes a gradient-index lens element that fulfills the condition shown below:
0 less than sgn[xcfx86G]xc2x7N1/xcfx86G2 less than 10.0
where
sgn[xcfx86G] represents a parameter whose value equals +1 when the optical power of the lens unit including the gradient-index lens element is negative and equals xe2x88x921 when the optical power of the lens unit including the gradient-index lens element is positive;
N1 represents the index distribution coefficient of the second order of the gradient-index lens element; and
xcfx86G represents the optical power of the lens unit including the gradient-index lens element.
According to another aspect of the present invention, in one of the above two zoom lens systems, the second lens unit includes a gradient-index lens element.
According to still another aspect of the present invention, in one of the above three zoom lens systems, the gradient-index lens element has at least one surface having a diffractive optical power (i.e. a diffractive optical""surface).